clang-format: [JS] do not clean up duplicated commas.

Summary:
In JavaScript, duplicated commas have semantic meaning.
    x = [a,,b];

The statement above creates an array with three entries, the middle being undefined. Because clang-format should not change semantics, disable this cleanup in JS.

Reviewers: djasper

Subscribers: klimek

Differential Revision: https://reviews.llvm.org/D33641

llvm-svn: 304141